1. Field of the Invention
The present invention relates to output drivers for integrated circuits (IC), and more particularly to low power, high performance output drivers designed to drive multiple-level switching partial-voltage signals.
2. Description of the Prior Art
Today, IC technologies have progressed into nanometer (nm) ranges. Current art 65 nm logic technologies provide transistors with switching time measured by 10−12 seconds (ps). It has become a routine practice to design logic circuits capable of executing billions or even trillions of operations per second. Such powerful core circuits require the supports of powerful interface circuits. Otherwise, input/output (I/O) bandwidth would become the performance bottleneck in high performance systems. It is therefore highly desirable to provide methods to improve the performance of I/O circuits for integrated circuits.
An output driver, by definition, is the last-stage circuit used to drive output signals from an IC to external components. A high performance output driver defined in this patent disclosure is the last-stage circuit used to drive high performance switching signals from an IC to external components while it is designed to support output signal switching rate higher than thousands, millions, or even billions of cycles per second. The performance of such output drivers has significant impacts to overall system performance.
The most common output drivers used by prior art IC are CMOS (complemented metal-oxide-semiconductor) drivers. CMOS drivers consume little power at steady state, and provide signals in full amplitude of I/O power supply voltage to represent digital data. However, switching noise related problems limited CMOS drivers in supporting high performance interfaces. It is therefore highly desirable to provide output drivers that can avoid switching noise problems to support high performance operations.
The most popular prior art method used to improve the performance of CMOS drivers is to reduce the amplitude of the output signals by introducing one or more termination resistor(s) to each signal line. The termination resistor is typically connected to a reference voltage equal to half of the I/O power supply voltage. The same reference voltage is also used for input data sensing. This method is called “high-speed transceiver logic” (HSTL) interface when it is used by high end SRAM (static random access memory) interface. A nearly identical method is also called “stub series terminated logic” (SSTL) interface when it is used by DRAM (dynamic random access memory) interface. These type of methods are called “small amplitude interfaces” (SAI) in our discussions. SAI effectively improved interface performance relative to conventional CMOS interfaces. However, SAI drivers consume power even when they are not switching data, and they still suffer most of the noise problems suffered by conventional CMOS drivers. It is therefore highly desirable to provide further improvements in performance relative to SAI while consuming little power at steady states.
Wireless devices such as cellular phones have progressed in explosive pace. Battery powered portable devices always require low power consumption. In the mean time, the demands for higher performance increase dramatically with each generation of wireless products. For example, cellular phones used to have no or very simple displays; now they require colored liquid crystal display (LCD) at high resolution. A current art LCD driver can send out 132 RGB signals (total 396 digital-to-analogy converter output signals) with 6 bit accuracy (64 levels) switching around 12 KHZ. Such IC devices require high accuracy, low power, digital-to-analog (D/A) output drivers. Most of prior art digital-to-analog converters use operation amplifiers with negative feedback to provide high accuracy output signals, but operation amplifiers typically consume a lot of power and have poor switching speed. Tsuchi disclosed an LCD driver design in U.S. Pat. No. 6,124,997 that does not use operation amplifiers; the method requires pre-charging each output line before driving a new data. The pre-charge operation will consume power no matter the data is changed or not. Since Tsuchi only use pull down driver, the method is sensitive to noises that cause the output signal to drop below targeted voltages. It is therefore highly desirable to provide low power output drivers that can support high accuracy switching signals.